so then if this is the correct equation my question... is how do you know when to consider the masses as a system and when not to?

net force = mass times acceleration

by deffintion
it dosen\'t mean this necessarily

net force = net mass times acceleration

when do I know when I should use the net mass and when not to as I have gotten points taken off before because i didn\'t use the net mass and am not sure when to consider the masses as one system and when not to...

I need help on when to make this distinction between the masses and when not to as I don\'t know when i should...

THANKS!!!
This has been eating me away and I need to know this or the whole problem is wrong so it\'s important that I know when to make this distinction and when not to. So can you tell me when I should and when I shouldn\'t???